I think the difference between the two is simply a matter of either making a decision based on information that isn't fully developed (interpretation), and going specifically against something written down to meet your playstyle.

So for the first one, I disagree with Yuki. Since it has no mention one way or the other about types of areas, and it does mention that it takes ambient moinsture, it seems to be simple interpretation of the rules to state it doesn't work in the desert. If the spell stated "This effect works even in low-moisture environments, such as deserts.", and the GM still didn't want it to work there, THAT's a house rule.

I agree the second is a house rule though. I find the majority of things that get changed are indeed interpretations, not house rules. House rules actively change the stated rules. I disagree with there being "few interpretations". Look at these boards and the WotC boards to see how many small things can be disagreed upon because of the wording in the books.